In many structural contexts, isogrids are used to add strength to a structure while minimizing the addition of weight. An isogrid is a type of partially hollowed-out structure formed usually from a single metal plate or face sheet with triangular integral stiffening ribs. It is extremely light and stiff. The triangular pattern in an isogrid is very efficient because it retains rigidity while saving material and therefore weight. The term isogrid is used because the structure acts like an isotropic material, with equal properties measured in any direction, and grid, referring to the sheet and stiffeners structure. The location where the ribs intersect is referred to as a node. An image of an isogrid is shown in FIG. 1.
A deficiency with isogrids arises when interconnecting two generally concentric circular structures. As used herein, the term circular structure will include structures that are generally circular in structure, such as a multi-sided polygon that approximates a circle, as well as columnar structures that are circular or approximately circular in cross-structure. The problem is that the nodes do not align uniformly with the circumference of the circular structures. Rather, the intersection of the circular structure and the isogrid is often somewhere along the length of a rib but not at the end of a rib and, therefore, not at a node. Accordingly, individual ribs must be faceted to the circular structure or false nodes must be added where the partial rib intersects with the circular structure. In either case, more structure is added, more weight is added and the performance of the isogrid is less than optimal. In addition, more design and manufacturing work is needed and labor costs increase due to additional faceting required for adding false nodes. FIG. 1 also illustrates how an isogrid intersects with a circular structure. As is readily apparent, very few nodes are located where the isogrid intersects with the outer circular structure. Although not shown in FIG. 1, isogrids similarly do not uniformly intersect with an inner circular structure at a node. At each location where a partial rib intersects the outer or inner circular structure, false nodes will need to be added in order to meet and maintain a structural design criteria.
One solution to the problem of interconnecting concentrically oriented circular structures is to utilize an orthogrid, which is a variant of the isogrid. An orthogrid uses rectangular rather than triangular openings. An orthogrid for circular structure is typically constructed in a radial pattern with a series of rectangles formed by radial and intersecting ribs increasing in area moving from the inner to the outer circular structure along an outwardly extending radial line. An orthogrid can be easier to manufacture and design than an isogrid, especially with circular structure, but an orthogrid is not isotropic in that it has different properties from different directions. An image of an orthogrid interconnecting two circular structures is shown in FIG. 2. As can be seen, the rectangles increase in size from the inner to the outer diameter. As a result, the ribs may need to be increased in size or shape at an outer radial position depending upon the intended loading.